This invention relates generally to turbo machinery and the like which is subject to high thrust forces which are created by unbalanced seal areas. More particularly, but not by way of limitation, this invention relates to thrust compensation apparatus for turbo machinery which utilizes magnetic bearings by providing thrust forces on the shaft to counteract the shaft thrust created by the case pressure.
In a single stage pipeline compressor, one end of the shaft is exposed to the full case pressure while the other end of the shaft is outside the pressure enclosure. This creates a very high thrust load on the shaft whenever the case is pressurized. In attempting to provide magnetic bearings to this type of compressor, two problems were found. The first problem is concerned with the load rating of magnetic bearings. Because of the limitations of the magnetic properties of available materials, magnetic bearings are limited to unit loads of 65 lb/sq. in. Thus, a 100 square inch magnetic bearing will handle up to 6500 lbs. of thrust. By comparison, an oil film bearing of the same size can accommodate 50,000 lb. on a continuous basis and up to 50% more for instantaneous conditions. For single stage units of this type, the thrust load at start up demands a magnetic thrust bearing which is so large, it is physically impractical, if not physically impossible to accommodate it. The second problem is concerned with the control of the shaft position. If the mass of the rotor is small in relation to the thrust bearing size, the inertia of the rotor will not be large enough to overcome the instantaneous corrections to the shaft position. Thus, the rotor will be overly sensitive to the control system and the proper shaft position cannot be maintained. Single stage compressor rotors tend to be very light in relation to the thrust load.
In order to utilize a magnetic thrust bearing in a single stage overhung compressor, it is necessary to develop a method of overcoming these two problems. Through the use of our invention, it is possible to reduce the high thrust load which exists at start up. The invention also makes use of some of the magnetic bearing features which are unavailable with oil film bearings. The inventive device is located in the central portion of the shaft where the bearings can better control the shaft movement. No external source of high pressure gas or fluid is required.